Sri Lanka today rejected a call by the UN for a ceasefire in its military campaign against the Tamil Tigers, insisting it would not be trapped into letting the group's leaders escape.

Rebels belonging to the Liberation Tigers of Tamil Eelam (LTTE) have taken refuge in a tiny enclave measuring less than eight square miles, alongside tens of thousands of civilians who are unable to leave.

Sri Lanka has accused the Tigers of using the civilians as human shields and claims they are being prevented from leaving. Aid agency officials say many civilians, including children, have been killed by the LTTE while trying to escape.

Yesterday a senior UN human rights official warned of a bloodbath unless tens of thousands of civilians are allowed to leave the war zone. Walter Kaelin, the representative of the UN secretary general, Ban Ki-moon, on the human rights of internally displaced persons, said: "If the Sri Lankan army would try to go into there, if the LTTE would not be ready to let these civilians go, then we'll end up with a bloodbath, and this must by all means be avoided."

More than 400 Tamil fighters have died in intense fighting since the start of the month. Many are understood to have been killed after refusing to surrender, despite running short of ammunition.

In London, the fighting prompted violent clashes between protesters and police today . Six people were arrested as a demonstration against the war entered its second day. Pro-Tamil demonstrators disrupted traffic on Westminster bridge until they were moved on by police to Parliament Square.

Todaythe Sri Lankan military acknowledged that it faced an "extremely difficult task" in separating the fighters from the civilians in the no-fire zone which it declared earlier this year for non-combatants to shelter from the fighting. It also accused the Tigers' leadership of attempting to manipulate international opinion to try to facilitate their own escape.

"They have merged into the rest of the civilians but they still have some strength to control those civilians," said Brigadier Udaya Nanayakkara, the Sri Lankan military spokesman. "They are forcefully keeping those civilians. They want us to move into that area and get international pressure to get asylum for their leadership. We don't want to get into that trap."

The Sri Lankan government is widely believed to be on the verge of a military victory against the Tigers, who been fighting for a separate state for ethnic Tamils in the country's north and east in a war that has lasted 25 years and claimed 70,000 lives.

But the military warned that even if it defeats the Tigers in their heartland, it could take another two years to finish off the sleeper cells which it says are present in towns and cities across the country.

"It is not going to end soon," said Nanayakkara. "They will resort to guerrilla tactics. It is already happening in the east.

"It will take time to completely eradicate terrorism from the country, we think about two years."

Many of the Tigers' most senior figures have been killed in heavy fighting since the start of the month, but the group's leader, Prabhakaran, remains at large.

Nanayakkara said Prabhakaran would be taken dead or alive.

"If we can capture him [alive] we will try, but if he comes with a weapon and if he fights then we will have to treat him as any other terrorist," he said.

Sri Lanka insists that its campaign against the Tigers is a humanitarian operation intended to free civilians who have been prevented from leaving the area. The rebel group accuses the government of shelling the zone, which the government strenuously denies.

Kaelin said more than 100,000 civilians were trapped in the conflict zone. The Sri Lankan government says the figure is no more than 40,000.

Kaelin called for a pause in the fighting to allow the trapped civilians to leave the area.

"I urgently repeat my call to the LTTE to allow all civilians under its control to leave this zone and to seek safety elsewhere," he said at the end of a four-day visit to Sri Lanka.

He also called on the government to "scrupulously respect" the no-fire zone.

Aid agencies fear that the trapped civilians are now in extreme danger as the military campaign reaches a climax.

The Sri Lankan president, Mahinda Rajapaksa, has repeatedly ruled out a ceasefire, saying it would allow the rebels to rearm and regroup, but the government has said it would continue to pause fighting – as it has done in the past – when civilians are in danger.